Hi BabyFugu,
Let me mention first that we run a Dive Centre based in Kota Kinabalu, Sabah. My aim is to share our experience with you to help you have the best time diving along the west coast Sabah, not to promote our diving to you.
We run PADI OW courses from Mantanani. My feedback is based on this experience.
Mantanani is like a 'desert island'. It has stunning beaches and, if the weather / sea state is kind, has clear blue waters. So great for snorkeling and good for diving along shallow coral reefs with nice blue, clear waters. There is a village on Mantanani (approx. 800 people), but there is little else there (no bars, restaurants, night life etc.). So how long you stay there depends up on how long you want to be away from it all.
It takes about 3 hours by minibus / boat transfer from central Kota Kinabalu to Mantanani. There are a couple of 'Resorts' on Mantanani, both are fairly basic.
You do need to be aware, for diving at Mantanani, that there has been destructive fishing practices around the islands so not all reefs are in the pristine condition they should be and the fish life is less than it should be. Having said that, if you are taken to the right dive sites the corals are nice and the visibility can be lovely. It all depends what you are looking for from your diving.
I hope this helps. Enjoy your diving
DG